1. Field of the Invention
The present invention relates to a device for detecting knocking in an internal combustion engine by using ionic current. More particularly, the invention relates to a device for detecting knocking of an internal combustion engine which, even when spike noise or noise due to disturbance of the flame in low-load running are generated, does not erroneously detect it as knocking.
2. Prior Art
In an internal combustion engine using gasoline as a fuel, a gas mixture compressed by a piston is ignited by a spark plug and is burned to produce an output. That is, in normal combustion, a flame nucleus in a gas mixture is formed near the gap of the spark plug, and propagates over the whole combustion chamber.
The ignition timing of the spark plug has an intimate relationship with the output of the internal combustion engine. When the ignition timing is too late, the propagation speed of the flame becomes slow. Therefore, the combustion becomes slow, resulting in a decrease in the combustion efficiency and, hence, in a decrease in the output of the internal combustion engine.
When the ignition timing is too early, on the other hand, the propagation of flame is fast, whereby the maximum pressure of combustion rises and the output of the internal combustion engine increases. When the ignition timing is too early, however, there takes place knocking in which the mixture gas is self-ignited prior to the propagation of the flame, often damaging the internal combustion engine.
That is, it is advantageous to operate the internal combustion engine in a region where the ignition timing is set just before the occurrence of knocking (MBT: minimum spark advance for best torque) from the standpoint of fuel efficiency and output. It is very important to reliably detect the occurrence of knocking.
A knock sensor which is a vibration sensor has heretofore been used for detecting knocking. However, a device has also been studied which detects knocking by utilizing the phenomenon that ions are generated in the combustion chamber due to the combustion of the mixture gas and an ionic current flows.
FIG. 1 is a diagram schematically illustrating an ignition circuit for an internal combustion engine, wherein an end of a primary coil 111 of an ignition coil 11 is connected to the positive electrode of a battery 12. The other end is grounded via the collector and the emitter of a switching transistor 13 included in an igniter.
The base of the transistor 13 is connected to an ignition timing control unit 14, so that the transistor 13 is turned on when an ignition signal IGT is output from the ignition timing control unit 14.
An end of a secondary coil 112 of the ignition coil 11 is also connected to the positive electrode of the battery 12, and the other end is connected to a spark plug 16 through a reverse current-preventing diode 15, a distributor (not shown) and a high-tension cable 18.
An ionic current detecting unit 17 is connected to the cathode of a reverse current-preventing diode 15 in parallel with the spark plug 16.
The ionic current is supplied, through a protection diode 171, to a series circuit of a current-to-voltage conversion resistor 172 and a bias power source 173. A voltage generated at a point where the current-to-voltage conversion resistor 172 and the protection diode 171 are connected together, is applied to an amplifier circuit 175 comprised of an operational amplifier and a resistor through a capacitor 174 for removing a DC component.
Therefore, a voltage signal proportional to the AC component of the ionic current is output at an output terminal 176 of the ionic current detecting unit 17.
FIGS. 2A to 2E are diagrams of waveforms at each of the portions of the ignition circuit (FIG. 1) and show, respectively, an ignition signal IGT, a voltage on the grounding side of the primary coil (point P), a voltage on the high-tension side of the secondary coil (point S), and a voltage of an amplifier circuit (point I). The abscissa represents time.
When the ignition signal IGT turns to the "H" level and the transistor 13 is turned on at t.sub.1, the voltage at point P drops. Immediately after t.sub.1, a negative high-voltage pulse is generated at point S, that is, on the high-tension side of the secondary coil. However, the current is blocked by the reverse current-preventing diode 15 from flowing into the spark plug 16 and the ionic current detecting unit 17.
When the ignition signal IGT turns to the "L" level at t.sub.2 and the transistor 13 is cut off, the voltage at point P abruptly rises, and a positive high-voltage pulse is generated at point S.
The positive high-tension pulse is not blocked by the reverse current-preventing diode 15 and flows into the spark plug 16 to be discharged. It is prevented by the protection diode 171 from flowing into the ionic current detecting unit 17.
Furthermore, from t.sub.3 to t.sub.4 after the discharge of the spark plug 16, LC resonance is triggered by energy remaining in the ignition coil 11 due to parastic inductance and parastic capacitance of the high-tension cable 18 and the like.
The gas mixture in the cylinder is ignited by the discharge of the spark plug 16, ions are generated in the cylinder as the flame spreads, and an ionic current starts flowing. The ionic current increases with an increase in the pressure in the cylinder and decreases with a decrease in the pressure in the cylinder.
When knocking occurs in the internal combustion engine, knocking signals in a particular frequency band (about 6 KHz) are superposed while the ionic current decreases after having reached its peak.
In order to detect the knocking using the ionic current, therefore, it is desired to detect only the knocking signals in a particular frequency band and reject other signals (e.g., LC resonance waves). For this purpose, therefore, it is desired to provide a knocking window which opens at t.sub.5 after spurious signals disappear and closes at a suitable moment (e.g., ATDC 60.degree.) after the ionic current has decreased, and to detect the knocking based upon the output of the ionic current detecting unit 17 while the knocking window is opened.
FIG. 3 is a diagram illustrating the constitution of a device for detecting knocking by using an ionic current. The output of the ionic current detecting unit 17 is supplied to a processing unit 34 through a band-pass filter (BPF) unit 32 and an integration (or peak-holding) unit 33. The operation of the integration (or peak-holding) unit 33 is controlled by a window which is opened after a predetermined period determined depending upon the engine speed and the load, and is closed at a moment corresponding to about 50.degree. CA.
In order to maintain the accuracy for detecting knocking irrespective of the conditions for detecting the ionic current, "a method of determining abnormal combustion and device therefor" which divides the ionic current signal into one part containing a relatively greater part of knocking frequency components and another part containing relatively smaller part of knocking frequency components, and detects knocking by comparing the ratio of one part to another with a predetermined reference level are already proposed (see Japanese Unexamined Patent Publication (Kokai) No. 61-57830).
However, the spike noise due to corona discharge of the spark plug 16 includes a wide frequency spectrum and affects the knocking frequency. Besides, the noise due to unstable combustion contains frequency components very close to the knocking frequency band. Therefore, it is not possible to accurately detect knocking by simply dividing the ionic current signal into two parts.
FIG. 4 is a diagram explaining the problems, and shows the waveforms of ionic current signals in the time domain and the frequency domain at a high-load normal combustion state, at an intermediate-load normal combustion state, at a state in which the knocking is taking place, at a state in which the spike noise is generated, and at a low-load normal combustion state.
That is, at the high-load and intermediate-load normal combustion states, the ionic current signals slowly increase and slowly decrease in the time domain when the knocking window is being opened. In the frequency domain, therefore, the level increases in the low-frequency side and decreases in the high-frequency side.
When knocking occurs, vibration components of about 6 KHz are superposed on the ionic signal while the knocking window is opened, and a peak appears near 6 KHz in the frequency domain.
When spike noise is generated while the knocking window is being opened, the level of the frequency domain rises as a whole so that it becomes difficult to extract a knocking peak from the ionic signal.
While operating at a low-load, furthermore, the flame in the combustion chamber is disturbed, and noise with a relatively low frequency spectrum is superposed on the ionic current. In the frequency domain, therefore, the level in the low-frequency band rises so that it becomes difficult to separate a knocking peak from the spectra.
The present invention provides a device for detecting knocking of an internal combustion engine, which does not erroneously detect knocking even when a spike noise or a noise due to disturbance in the flame in the low-load zone is generated.